1. Field of the Invention
The invention pertains to the field of semiconductor devices. More particularly, the invention pertains to ultrahigh-speed optoelectronic devices, such as light-emitting diodes and laser diodes.
2. Description of Related Art
High-speed optoelectronic devices are broadly applied in modern datacommunication and telecommunication systems.
These devices can be separated into two categories: those directly modulated by injection of current into the gain region, and those externally modulated. Direct modulation offers the advantage of low cost but requires very high photon densities in the resonant cavity. For, example, edge-emitting lasers operating at 40 Gb/s have been reported.
The intrinsic speed is defined by the so-called “−3 dB” bandwidth, which is roughly proportional to the relaxation-oscillation frequency:
                                          f            r                    =                                    1                              2                ⁢                π                                      ⁢                                                                                g                    n                                    ⁢                                      p                    0                                                                    τ                  p                                                                    ,                            (        1        )            where gn denotes the differential gain, p0 is the average photon density in the cavity, and τp is the cavity photon lifetime.
A first way to increase the laser bandwidth is to increase the pump current density thereby increasing the photon population of the cavity, for example, by reducing the surface area of the device for the same total current. Under pulsed excitation relaxation, an oscillation frequency as high as 70 GHz has been demonstrated in a pulsed regime at room temperature under applied voltage of 15 volts. The problem for direct modulation is the overheating of the active region in the continuous wave regime and the related saturation of the differential gain with current and the related saturation of the relaxation oscillation frequency. Another challenge for direct modulation is degradation stability of the device. At the very high current densities the degradation rate may be unacceptably high.
Another big problem of direct modulation is a high differential capacitance of the device under forward bias. The injected carriers reduce the effective thickness of the undoped layer in the p-n-junction and increase the capacitance. Thus, realization of ultrahigh-speed devices is challenging also in this case.
In contrast, indirect modulation using elecrooptic effects under reverse bias has long been known in ultrahigh-speed transmitters operating at 40-60 Gb/s. For example, a 40-Gb/s open eye diagram of the electroabsorption modulator after 700-km transmission has been demonstrated.
Once the need for direct modulation is abandoned, ultrahigh-speed signal management becomes much easier. 60-100 GHz pin diode photodetectors using large mesa devices as well as other devices are known in the art.
U.S. Pat. No. 6,285,704, “FIELD MODULATED VERTICAL CAVITY SURFACE-EMITTING LASER WITH INTERNAL OPTICAL PUMPING”, issued Sep. 4, 2001, proposes a photopumped VCSEL. This VCSEL may be modulated by using an external electrical field applied perpendicular to the active layer, employing the Stark-effect to deliberately change the bandgap of the active layer and hence move the emission wavelength into and out of resonance with the optical cavity formed between the top and bottom mirrors. The optical output is therefore modulated by the electrical field and not by injected carriers. However, as the active region of the device is under a continuous population inversion condition, applying a reverse bias to change the bandgap may cause dramatic photocurrent, depleting the photopumped active region.
U.S. Pat. No. 5,574,738, “MULTI-GIGAHERTZ FREQUENCY-MODULATED VERTICAL-CAVITY SURFACE-EMITTING LASER”, issued Nov. 12, 1996, discloses a saturable absorber contained within the VCSEL's distributed Bragg reflector, which may itself be adjusted during fabrication or in operation. Under controllable operating conditions, the saturable absorber, strategically sized and placed, forces the VCSEL to self-pulsate (in the GHz-regime) at rates related to the local intensity, absorption, lifetime, and carrier density of the saturable absorber. In one of the embodiments, efficiency of the saturable absorber may be controlled by the quantum-confined Stark effect. Mode-locked operation, however, is usually very sensitive to the conditions of the device operation and exists only in a relatively narrow range of carefully-optimized conditions.
U.S. Pat. No. 6,396,083, entitled “OPTICAL SEMICONDUCTOR DEVICE WITH RESONANT CAVITY TUNABLE IN WAVELENGTH, APPLICATION TO MODULATION OF LIGHT INTENSITY”, issued May 28, 2002, discloses a device including a resonant cavity. The resonant cavity is delimited by two mirrors and at least one super-lattice that is placed in the cavity and is formed from piezoelectric semiconducting layers. The device also includes means of injecting charge carriers into the super-lattice. One disadvantage of this device is the necessity of using piezoelectric materials. The piezoelectric semiconducting layers are epitaxially grown on a Cd0.88Zn0.12Te substrate and include a pattern composed of a layer of Cd0.91Mg0.09Te and a layer Cd0.88Zn0.22Te, each 10 nm thick. This pattern is repeated about a hundred times. The device in this patent is a two-terminal device. The separation of carriers in a piezoelectric superlattice causes long depopulation times. Wavelength modulation and intensity modulation are always interconnected in this patent.
An electrooptic modulator based on the quantum confined Stark effect (QCSE) in a VCSEL was disclosed in U.S. Pat. No. 6,611,539, “WAVELENGTH-TUNABLE VERTICAL CAVITY SURFACE-EMITTING LASER AND METHOD OF MAKING SAME” issued Aug. 26, 2003, by the inventors of the present invention and herein incorporated by reference. The device includes active media suitable for providing gain and enabling laser action of the device, and a position-dependent electrooptic modulator region. Applying the voltage to the modulator region results in a wavelength shift of the lasing wavelength. The absorption in the modulator region remains small. The device is especially applicable for ultrahigh-speed data transfer using wavelength-modulation.
U.S. Patent Publication 2003/0206741, entitled “INTELLIGENT WAVELENGTH DIVISION MULTIPLEXING SYSTEMS BASED ON ARRAYS OF WAVELENGTH TUNABLE LASERS AND WAVELENGTH TUNABLE RESONANT PHOTODETECTORS”, published Nov. 6, 2003, by the inventors of the present invention and herein incorporated by reference, disclosed high-bit rate data transfer systems based on wavelength-to-intensity modulation conversion. In this approach, a wavelength-tunable VCSEL operates in concert with a wavelength-selective photodetector on the receiver side. Modulation of the VCSEL wavelength transforms into the photodetector current modulation.
U.S. Patent Publication 2005/0271092, entitled “ELECTROOPTICALLY WAVELENGTH-TUNABLE RESONANT CAVITY OPTOELECTRONIC DEVICE FOR HIGH-SPEED DATA TRANSFER” published Dec. 8, 2005, by the inventors of the present invention and herein incorporated by reference, disclosed high-bit rate data transfer system based on a device, which contains at least one wavelength-tunable element controlled by an applied voltage and at least two resonant cavities. The resonant wavelength of the tunable element is preferably elecrooptically tuned using the quantum confined Stark effect around the resonant wavelength of the other cavity or cavities, resulting in a modulated transmittance of the system. A light-emitting medium is preferably introduced in one of the cavities permitting the optoelectronic device to work as an intensity-modulated light-emitting diode or diode laser by applying an injection current. The device preferably contains at least three electric contacts to apply forward or reverse bias and may operate as a vertical cavity surface-emitting light emitter or modulator or as a tilted cavity light emitter or modulator. The problem of this device, however, is need in very strict growth tolerances, as the device operation is extremely sensitive to the spectral position of the cavity mode of the wavelength-tunable resonating cavity with respect to the VCSEL cavity mode. Assuming the growth rate non-uniformity for different materials used in the modulator and the VCSEL sections, there is unavoidable non-uniformity in device performance across the wafer. Another disadvantage is the fact that the output power is a non-monotonous function of the applied voltage. The device has low power in the absence of the applied voltage (the cavities are out of resonance), the power is high at a certain voltage (the cavities are in resonance), and the power is again low at even higher bias voltages (the cavities are out of resonance).
As the standard telecom and datacom devices operate only in the “on-off” mode, this non-monotonous characteristic is highly undesirable.
Thus, there is a need in the art for a robust ultrafast way to modulate the intensity of the device.